Saving Sunshine

The standard knock on solar power has been, what if the sun isn't shining? Schemes to make solar a large-scale energy source have focused on developing a battery that can keep power coming at night or when skies are overcast. But batteries are expensive. Now some companies are touting solar thermal devices, which store energy as steam in a pressurized vessel, as a less costly alternative. Ausra, a maker of solar thermal plants, has developed a technology that uses mirrors to focus light onto tubes filled with water, producing steam to drive a turbine. The liquid storage typically retains 95 percent of the solar energy, in contrast to 65 percent for an electric battery, says John O'Donnell, executive vice president. The plant, he says, can produce electricity as cheaply as coal, oil or gas plants. Solar thermal plants are now catching on in the United States, Germany and Spain.

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